The present invention generally relates to a process for substantially reducing undesirable flavor components in vegetable protein materials, specifically vegetable protein isolates.
Protein isolates that are derived from vegetable protein sources such as the oilseeds, typically soybeans, cottonseed and the like, have contributed substantially to the economic importance of these vegetable protein materials as a crop. Soy isolate has particularly proven to be a useful and beneficial nutritional supplement in a variety of food and beverages. A protein isolate can be generally characterized as a product resulting from the extraction, subsequent concentration and purification of the proteinaceous material from a proteinaceous source such as a vegetable protein or oilseed material. Typically a protein isolate derived from a vegetable protein source such as soybeans will have a protein content on a moisture free basis that will range between about 90 and 98% by weight after isolation of the protein from the non-proteinaceous components in the vegetable protein material.
A conventional procedure for the isolation of protein from a vegetable protein material such as an oilseed meal or flakes, comprises as the first step, removing soluble lipids or oil from the vegetable protein material, either by means of a mechanical apparatus such as an expeller or by solvent extraction with an aliphatic hydrocarbon such as hexane. The material obtained from solvent extraction of the oilseed or vegetable protein source, is commonly referred to as defatted oilseed meal and is normally in the form of flakes which will contain many ingredients including complex and varying types of proteins, sugars and fiber. In a conventional isolate process, the protein and sugars are dissolved from the flakes by dispersing the flakes in an aqueous bath and adding a food grade alkaline material to raise the pH of the mixture to substantially greater than 7 in order to expedite removal of the protein. Typical alkaline materials which are used for this purpose include sodium, potassium or calcium hydroxide. The extract containing the dissolved protein and sugars is then separated from insoluble solids by centrifugation.
The pH of the clarified extract is then reduced to between about 4 and 5 in order to lower the pH of the mixture to near the isoelectric point of the protein. The protein immediately begins to precipitate from the solution as a "curd" which is separated by centrifugation or filtration of the solution containing the curd or precipitate in order to concentrate the curd for further processing.
Soy isolates in spite of their desirable nutritional properties have not always had the most desirable flavor for various types of food applications. Accordingly, a great deal of effort has been devoted towards improving the functionality or flavor of protein isolates such as soy isolates. U.S. Pat. No. 3,642,490, for example, describes a process wherein a slurry of the precipitated or isolated soy protein is subjected to dynamic heating conditions at an essentially neutral pH in order to improve the flavor and the dispersibility characteristics of the isolate.
It has also been recognized that an important factor in the development of undesirable flavors in soybeans is the oxidation of native lipids contained in the soybean material. Oxidation of lipid material can occur because of the presence of a group of enzymes identified as lipoxygenase, which typically catalyze the oxidation of lipids in the soybean thereby producing hydroperoxide compounds which can in turn undergo further transformation by enzymatic and nonenzymatic reactions to give a wide variety of compounds that adversely effect the flavor of the soybean product. Among the many components that have been identified in the volatile fraction from soy milk is the compound n-hexanal which represents about 25% of the total volatile fraction obtained from soy milk. A number of compounds besides n-hexanal have also been identified in the volatile fraction from soy milk and it is believed that many of these compounds contained in the total volatile fraction adversely effect the flavor of the soy material.
Although a number of procedures have been proposed for reducing the level of the lipoxygenase in the native soybean used as the starting material, nevertheless a means of reducing the inherent flavor in soy products without specific control of the starting material is still needed. It has long been recognized for example, that the fermentation of soy products with various molds and bacteria have been shown to decrease the "beany" off flavors contained in soy products. For centuries, a fermented soy food-stuff called "Tempeh", a traditional Indonesian food product, has been recognized as a product in which the "beany" off flavors associated with soybeans has been substantially reduced because of the fermentation process.
U.S. Pat. No. 3,917,851 for example, describes such a fermented soy protein product in which a soy protein material having a fat content not exceeding 2% by weight, is fermented in an aqueous medium with a mold of the genus Rhizopus and the fermented material is then recovered from the medium. The recovered material has an improved odor and flavor. U.S. Pat. No. 4,216,235 also describes a fermentation process in which an aqueous soy suspension is fermented with a strain of S. cerevisiae for purposes of reducing the flatulent sugars present in soy products.
It is therefore apparent that a number of fermentation processes have been proposed for improving the odor, flavor or digestibility of soy products. In spite of the noted procedures, fermentation processes require an extensive amount of time, usually 24 hours or longer. Furthermore, most fermentation processes substantially alter the functional or physical characteristics of the soy protein. It would therefore be desirable if a means could be found for reducing the objectionable flavors or odors contained in soy protein which involves a minimal amount of time but without substantially changing the functional or physical characteristics of the protein.
This objective has been obtained in the present invention wherein it has been determined that an aqueous soy suspension may be contacted with a mold of the genus Rhizopus or genus Aspergillus, under conditions such that substantial fermentation is avoided, yet objectionable flavor and odor components are substantially reduced.
It is therefore an object of the present invention to provide a process for the reduction of undesirable odors and flavors contained in a vegetable protein material, specifically soy protein.
It is a further object of the present invention to provide a process for the reduction of odors or flavors contained in a vegetable protein isolate, specifically soy isolate.
It is a still further object of the present invention to provide a process for the reduction of undesirable flavors and odors contained in a soy protein isolate by a process that does not substantially alter the physical or functional characteristics of the soy protein.